Low total testosterone (TT) is present in about 30% of men aged >60 years and in up to 7% of younger men. Male hypogonadism is associated with metabolic and cardiovascular diseases as well as with increased mortality. There is evidence showing a relationship of TT with vitamin D in men. We aim at evaluating the effect of vitamin D supplementation on TT and metabolic parameters in hypogonadal men. We will study the effects of 20,000 IU vitamin D weekly in a 12 wk randomized, double-blind, placebo-controlled trial in 100 men with TT <3.0 ng/ml and 25-hydroxyvitamin D (25(OH)D) <30 ng/ml (patients) as well as in 100 men with TT ≥3.0 ng/ml and 25(OH)D <30 ng/ml (controls). Vitamin D supplementation might be a safe therapeutic approach improving TT levels as well as metabolic parameters in hypogonadal men. Further the effects of vitamin D on androgens will be evaluated in eugonadal men.